A liquid crystal display device is thin and light, therefore, has been widely used as a display for a personal digital assistant. The liquid crystal device does not emit light itself and is a device to express the display by changing the transmissivity and can be driven with several volts of effective voltage. Accordingly, when the liquid crystal display device is used as a reflective one which comprises a reflector provided at the back side of the liquid crystal cell and expresses the display by reflecting the outside light, a display device with an extremely low consumption of electricity can be provided.
A conventional reflective color liquid crystal display device comprises a liquid crystal cell having a color filter and a pair of polarization films by which the liquid crystal cell is sandwiched. In this case, the color filter is provided on one of the substrates of the liquid crystal cell and a transparent electrode is formed on the color filter provided on the substrate. A voltage is applied to the liquid crystal cell to change the alignment of liquid crystal molecule. Accordingly, the transmissivity of each color filter is changed so as to express a color display.
The transmissivity of one polarization film is 45% at most, as a whole. In this case, the transmissivity of the polarization component parallel to the adsorption axis of the polarization film is about 0% and the transmissivity of the polarization component perpendicular to the adsorption axis of the polarization film is about 90%. Accordingly in a reflective liquid crystal display device comprising two polarization films, when the outside light is incident and reflected by a reflector and goes outside, the light passes through the polarization film four times. As a result, when the transmissivity of the polarization component perpendicular to the adsorption axis of the polarization film is 50% and the light is not adsorbed by the color filter, the reflectance of the outside light is obtained by the following formula. EQU 0.9.sup.4.times.50%=32.8%
Accordingly, when the color filter of the conventional reflective color liquid crystal display device is removed and used as a black and white panel, in the same way, the reflectance reaches 33% at most.
In order to brighten the light display, several reflective liquid crystal display devices comprising one polarization film provided on the upper side of a liquid crystal cell that is sandwiched by the polarization film and the reflector have already been proposed. (refer to Japanese Laid-Open Patent Publication No. 7-146469 and No. 7-84252). In these proposed reflective liquid crystal display devices, the light passes through the polarization film only twice. When the light is not adsorbed by the color filter, the reflectance of the outside light is obtained by the following formula. EQU 0.9.sup.2.times.50%=40.5%
Therefore, these proposed reflective liquid crystal display devices can be expected to increase the reflectance by at most about 23.5% in comparison with that of the liquid crystal display device comprising two polarization films.
A reflective color liquid crystal display device in which color display is expressed by using the birefringence of a twist orientation nematic liquid crystal and polarization film, without using a color filter, has been disclosed in Japanese Laid-Open Patent Publication No. 6-308481. A color liquid crystal display device in which birefringence of liquid crystal and phase difference film is used has been disclosed in Japanese Laid-Open Patent Publication No. 6-175125 and No. 6-301006.
In the reflective liquid crystal display device which expresses color display with color filter and comprises one polarization film to increase the reflectance and to obtain the brightness, however, it is difficult to express a black and white display in achromatic color, especially it is difficult to express a black display in achromatic color with low reflectance. Further, in this reflective liquid crystal display device, the dependency of the reflectance and brightness on the incident direction of the outside light and the observer's viewing direction, that is, the visual angle dependency of the optical characteristics, is high. When the visual angle dependency of the reflective liquid crystal display device comprising one polarization film is high, the visual angle is narrow and the following problem also occurs. That is, when black brightness is changed greatly with respect to the change of incident direction of the outside light, the optical characteristic of the reflective liquid crystal display device is deteriorated greatly since it is more difficult for the reflective liquid crystal display device to control the incident direction of the outside light in comparison with the transmissive liquid crystal display device.
In the reflective color liquid crystal display in which a color display is expressed by using birefringence of a twist alignment nematic liquid crystal and polarization film, without using a color filter or the color liquid crystal display in which birefringence of liquid crystal and phase difference film is used, since the color filter is not present, the efficient reflectance to obtain the practical brightness can be ensured even if two polarization films are used. However, in the above-mentioned color liquid crystal display devices, since the color display is expressed by using birefringence, it is difficult to express multi-grade and multicolor display such as 16 grade 4096 color display. Further, the range of color purity and color reproduction is also narrow.
In the reflective liquid crystal display device with black and white mode comprising two polarization films, a white display with high reflectance can't be obtained and the display becomes dark.